it’s a lot of code to learn.
There are hundreds of filters and actions.
most of it is detective work.
I have bbpress in a folder on my laptop/
I use
https://notepad-plus-plus.org/
as my main tool, it understands all the file types and displays them in a very readable way.
It has a ‘search on files’, so in your case I looked for ‘subscribe’ as this was the word that had the | before it.
that got me to the subscribe link function, which has the ' | ' in it, which got me to the filter
When you came back with it changing when you click, I then searched for ' | ' and found it in the core function.
For the ‘before_xx_parse_args’ and other filter stuff see